In conjunction with the recent rapid advances of portable electronic equipment and communications instruments, nonaqueous electrolyte secondary batteries having a high energy density are strongly demanded from the aspects of cost, size and weight reductions. Approaches known in the art to increase the capacity of such nonaqueous electrolyte secondary batteries include, for example, use as negative electrode material of oxides of B, Ti, V, Mn, Co, Fe, Ni, Cr, Nb, and Mo and composite oxides thereof (JP 3008228 and JP 3242751: Patent Documents 1 and 2); application as negative electrode material of M100-xSix wherein x≧50 at % and M=Ni, Fe, Co or Mn which is obtained by quenching from the melt (JP 3846661: Patent Document 3); use as negative electrode material of silicon oxide (JP 2997741: Patent Document 4); and use as negative electrode material of Si2N2O, Ge2N2O or Sn2N2O (JP 3918311: Patent Document 5).
Silicon is regarded most promising in attaining the battery's goals of size reduction and capacity enhancement since it exhibits an extraordinarily high theoretical capacity of 4,200 mAh/g as compared with the theoretical capacity 372 mAh/g of carbonaceous materials that are currently used in commercial batteries. Silicon is known to take various forms of different crystalline structure depending on a preparation process. For example, JP 2964732 (Patent Document 6) discloses a lithium ion secondary battery using single crystal silicon as a support for negative electrode active material. JP 3079343 (Patent Document 7) discloses a lithium ion secondary battery using a lithium alloy LixSi (0≦x≦5) with single crystal silicon, polycrystalline silicon or amorphous silicon. Of these, the lithium alloy LixSi with amorphous silicon is preferred, which is prepared by coating crystalline silicon with amorphous silicon resulting from plasma decomposition of monosilane, followed by grinding. However, the material therein uses 30 parts of a silicon component and 55 parts of graphite as the conductive agent as described in Example, failing to take full advantage of the potential battery capacity of silicon.
Methods known to impart electric conductivity to negative electrode materials include mechanical alloying of a metal oxide such as silicon oxide with graphite and subsequent carbonization (JP-A 2000-243396: Patent Document 8); coating of Si particles on their surface with a carbon layer by chemical vapor deposition (JP-A 2000-215887: Patent Document 9); and coating of silicon oxide particles on their surface with a carbon layer by chemical vapor deposition (JP-A 2002-42806: Patent Document 10). The provision of particle surfaces with a carbon layer improves conductivity, but is not successful in overcoming the outstanding problems of silicon negative electrodes, i.e., in mitigating substantial volumetric changes associated with charge/discharge cycles or in preventing degradation of electrical conductivity and cycle performance.
Recent approaches taken for this reason include a method for restraining volume expansion by restricting the percent utilization of silicon battery capacity (JP-A 2000-215887, JP-A 2000-173596, JP 3291260, and JP-A 2005-317309: Patent Documents 9, 11 to 13), a method of quenching a melt of silicon having alumina added thereto for utilizing grain boundaries in polycrystalline particles as the buffer to volumetric changes (JP-A 2003-109590: Patent Document 14), polycrystalline particles of mixed phase polycrystals of α- and β-FeSi2 (JP-A 2004-185991: Patent Document 15), and hot plastic working of a monocrystalline silicon ingot (JP-A 2004-303593: Patent Document 16).
Means for mitigating volume expansion by tailoring the layer structure of silicon active material are also disclosed, for example, disposition of two layers of silicon negative electrode (JP-A 2005-190902: Patent Document 17), and coating or encapsulating with carbon or another metal and oxide for restraining particles from spalling off (JP-A 2005-235589, JP-A 2006-216374, JP-A 2006-236684, JP-A 2006-339092, JP 3622629, JP-A 2002-75351, and JP 3622631: Patent Documents 18 to 24). Also disclosed is a method of gas phase growing silicon directly on a current collector wherein degradation of cycle performance due to volume expansion is restrained by controlling the growth direction (JP-A 2006-338996: Patent Document 25).
The method of enhancing the cycle performance of negative electrode material by coating silicon surfaces with carbon to be electrically conductive or coating silicon with an amorphous metal layer as mentioned above utilizes only about a half of the silicon's own battery capacity. There is a desire for a higher capacity. As for the polycrystalline silicon having grain boundaries, the disclosed method is difficult to control the cooling rate and hence, to reproduce consistent physical properties. There is a desire to have a negative electrode active material which can restrain the volumetric change associated with occlusion and release of lithium, mitigate a lowering of conductivity due to pulverisation by crack of particles and separation of particles from the current collector, be manufactured on a mass scale at a low cost, and comply with the application as in mobile phones where repetitive cycle performance is of high priority.
Patent Document 1: JP 3008228
Patent Document 2: JP 3242751
Patent Document 3: JP 3846661
Patent Document 4: JP 2997741
Patent Document 5: JP 3918311
Patent Document 6: JP 2964732
Patent Document 7: JP 3079343
Patent Document 8: JP-A 2000-243396
Patent Document 9: JP-A 2000-215887
Patent Document 10: JP-A 2002-42806
Patent Document 11: JP-A 2000-173596
Patent Document 12: JP 3291260
Patent Document 13: JP-A 2005-317309
Patent Document 14: JP-A 2003-109590
Patent Document 15: JP-A 2004-185991
Patent Document 16: JP-A 2004-303593
Patent Document 17: JP-A 2005-190902
Patent Document 18: JP-A 2005-235589
Patent Document 19: JP-A 2006-216374
Patent Document 20: JP-A 2006-236684
Patent Document 21: JP-A 2006-339092
Patent Document 22: JP 3622629
Patent Document 23: JP-A 2002-75351
Patent Document 24: JP 3622631
Patent Document 25: JP-A 2006-338996